1. Field of the Invention
The present invention relates generally to a plasma diagnosing apparatus, and more specifically to a plasma diagnosing apparatus capable of detecting and removing contamination of the probe.
2. Description of Related Art
In recent years, the plasma processing techniques such as plasma CVD, plasma polymerization, etc., have been widely adopted in various fields of technology. Since most of these techniques employ the reactive plasma, a problem will arise as to contamination of a probe at the time of diagnosing the plasma using such probe.
Recently, not only the basic study in the plasma has made its advancement, but also its applications in various industrial fields have been broadened. As the result, it has become necessary to diagnose instantaneously from outside various parameters within the plasma, in particular its electron temperature and electron density. The so-called "triple probe direct viewing method", as the method of diagnosis, is the most effective way, with which these parameters are instantaneously indicated on the gauges.
In general, there are two principal methods of diagnosing the plasma parameters: the one is a probing method, by which electrodes are directly intromitted into the plasma to take out electric current from it, and then the current is analyzed to find out the parameters of the plasma; and the other is an electromagnetic wave method, by which microwaves, lasers, and others are put into the plasma to directly sense out the results of their interaction, or to detect, by spectroscopy, a beam to be emitted from the plasma, to thereby find out these parameters.
While this latter method, i.e., the electromagnetic wave method is excellent in its horological resolution, it has disadvantages such that the device becomes complicated in its structure, hence expensive in its manufacture. In contrast to this, the former method, i.e., the probing method is characteristically excellent in its spatial resolution, simple in the device construction, and can be manufactured at a low cost, although the plasma to be diagnosed is limited to one having a relatively low temperature and low density.
In general, since most of the plasma used in laboratories, etc. is of relatively low temperature and low density, probing method is the fundamental for diagnosing the plasma having its electron density Ne of as low as 10.sup.14 cm.sup.-3 or below and its electron temperature Te of as low as a few tens of electron volts (eV) or below.
In case the parameters of the reactive plasma are to be diagnosed by the probing method, the surface of the probe becomes covered with a contaminant film as the time passes by, with the consequence that the exposed part of the probe is required to be cleaned.
There has heretofore been adopted a method for cleaning the probe with it being installed in the vacuum device, wherein, after a reactive gas is exchanged for an inactive gas as the case may be, a negative voltage is imparted to the probe with respect to a reference electrode to bring it to a red heat condition by the ion bombardment for removal of the contaminant film.
When, however, the ionic energy increases, the heat generation on the surface of the probe also increases, so that, for avoiding destruction of the probe due to heating, the voltage and other parameters should be controlled with care. The manual control of these parameters was fairly difficult and cleaning of the probe was often unsatisfactory, or the probe becomes molten due to excessive voltage application, hence good result could not always be obtained.
In the diagnosis of the reactive plasma by means of the conventional plasma diagnosing apparatus, if the reactive plasma is silane gas plasma, for example, an amorphous silicon film is produced on the surface of the probe with passage of time, and, if the plasma is styrene plasma, a styrene polymer film is produced on the surface of the probe. On account of this, the voltage-current characteristic of the probe changes with growth of the film, its shape becomes distorted, and the measurement finally becomes impossible.
When the abovementioned conventional plasma diagnosing probe was used, no reliable results of measurement could be obtained, because there was no quantitative method of detecting cleanliness of the probe, on account of which the probe had to be used without warranty of its cleanliness, and further, magnitude, imparting time, etc. of the negative voltage to be imparted to the probe at the time of removing the contaminant film by the ion bombardment were irregular and not consistent. As to the contamination detecting method, trials have so far been done in some area such as observation of the ionosphere by a rocket, wherein an alternating current is applied to the probe in a thin plasma to detect contamination of the probe in utilization of the hysteresis phenomenon due to the polarity in the electric potential. This detection method, however, was limited only to such thin plasma, and there were also many uncertain factors such that, depending on the type of contaminants, no hysteresis phenomenon appears, or inversely, depending on electric potential, frequency for voltage application, etc. , the hysteresis phenomenon varies, and so forth. As the consequence, it was difficult to adopt this contamination detection method as the general method for detecting the contamination of probe under extensive plasma conditions.
Also, as to cleanliness of the probe, the applicant has already proposed, in his prior patent application No. 149221/1989 (Laid-Open Patent Application No. 15197/1991), a method for automatic cleaning of a probe for measuring the plasma parameters. The invention is directed to obtain an appropriate cleaning effect with a ratio between the measuring time (diagnosing time) and the cleaning pulse width being made constant.
However, while the cleaning method is effective in the case where the diagnosing time using the voltage-current characteristic is long, as is the case with the general probing method, it is not necessarily effective for the triple probing method.
That is to say, the triple probe method is one which carries out the diagnosis by first placing three probes, each having an equal area, in mutually adjacent relationship at positions in the plasma where the equipotential seems to exist, and then by applying electric voltages of different potentials to each of these probes. While this method is an excellent one capable of directly reading the parameters of the plasma instantaneously, it is still not suitable for the abovementioned cleaning method, because of shortness in its diagnosing time.